1. Field of the Invention
The present invention relates to a method of charging a battery array that is a plurality of batteries connected in series, and in particular relates to a method that is optimal for charging a battery array that is a plurality of lithium ion rechargeable batteries connected in series.
2. Description of the Related Art
When a battery array, which is a plurality of batteries connected in series, is charged, each battery is charged by the same charging current. Therefore, if the electrical characteristics of all the batteries are identical, each battery will be charged to the same voltage. However, in an actual battery array, the voltage of each battery does not become the same. This is because the electrical characteristics of all battery cells cannot be made completely the same. The voltage difference of each battery relative to others becomes larger with use. This is because the relative imbalance in degradation of each battery increases with use. This drawback can be eliminated by a method that charges each series connected battery independently. However, a charging circuit for this method becomes complex, and since it is necessary to expose each battery connecting node externally as a terminal, there is no way the method can be adopted in actuality. Further, a practical battery array of this type has not been developed. As a result, a battery array is charged by connection of its positive and negative output terminals to a battery charger. Consequently, voltage differences develop due to the relative imbalance of the batteries.
If the voltage of a particular battery exceeds a maximum specified voltage during charging, degradation of that battery becomes significant, and safely charging the battery array becomes impossible. For this reason a method that charges a battery array while detecting the voltage of each battery and suspends charging when the voltage of any battery exceeds a maximum specified voltage is cited in Japanese Patent Application Disclosure 2001-126772